The present invention relates to a process and to an apparatus having a modular structure for the distillation of a fluid. The present invention has particular applicability to the desalination of briny water and/or sea water, although other applications are also possible.
Known desalination techniques can be classified according to three large families, depending on the process which is applied. These families include processes involving a change of phase (namely, freezing and distillation), processes using membranes (namely reverse osmosis and electrodialysis), and processes acting on chemical bonds (namely ion exchange, extraction by solvents and the formation of hydrates).
Distinguished among the most tried and tested techniques that have reached an industrial stage are the membrane techniques, such as reverse osmosis, electrodialysis and multistage flash distillation or multiple-effect distillation processes.
Reverse osmosis is a process for separating water from dissolved salts by means of membranes due to the action of pressure. This process operates at room temperature.
Electrodialysis is a process which demineralizes briny water by migration of the various ions through selective membranes which are subjected to the action of an electric field. The electric field results from the application of a DC potential difference between two electrodes delimiting the selective membranes.
A multistage flash distillation process is based on the distillation of sea water which has been preheated to a temperature which varies between 80 and 120xc2x0 C. This distillation derives from successive expansions of the sea water within a series of cells, and is based on the principle of vacuum evaporation of part of the sea water which has been preheated to a temperature which varies between 70 and 80xc2x0 C. This evaporation, which takes place on an exchange surface, unlike the previous process which takes place by expansions within the cells, results from the transfer of heat given up either by condensation of the steam (latent heat) coming from a boiler or by the cooling of hot water from a heat recuperator.
The technique employed in accordance with the present invention falls within this latter category, and is based on falling-film exchanger technologies in which films run down multiple-effect plates. A multi-usage flash evaporator of this type is described, for example, in French Patent Application FR 2 666 517.
Water production plants falling within this latter category are very widely used throughout the world. However, the overall investment and operating costs of such facilities remain higher than those of the other two types of processes. More than 50% of the cost of the overall investment is tied up in the distillation unit itself.
The present invention therefore has as its objective to provide a distillation unit of an entirely novel structure so as to significantly reduce the cost of the plant.
To achieve this objective, a modular distillation unit is provided which operates on a novel process and which results from the assembly of a large number of simple elementary components which are either commercially available or easy to manufacture in high volume and for a low cost (even in low-technology countries).
To achieve this, an elementary evaporation/condensation cell for a falling-film distillation unit based on films running down multiple-effect plates is provided. The volume of the structure is bounded by two metal plates forming the walls of the cell. The plates are held vertically and are separated by a spacer frame. The spacer frame is divided by a longitudinal central member into two distinct zones which are substantially identical and which communicate with each other, which will hereafter be called the evaporation zone and the condensation zone, respectively.
The evaporation zone is fed with fluid to be distilled via a feed circuit which enters the upper part of the evaporation zone. The fluid to be distilled runs down the two walls of the cell, as a thin film. Excess unevaporated liquid or concentrate is recovered in the lower part of the evaporation zone via a concentrate circuit, and the distilled fluid is recovered in the lower part of the condensation zone via a distillate circuit. The evaporation and condensation zones communicate with each other via a series of calibrated holes provided in the central member. An anti-collapse device is optionally provided inside the cell if the cell is to be maintained at a reduced pressure.
In a first embodiment, a manifold is defined by a transverse member which is drilled with a plurality of calibrated holes and which is fastened in the upper part of the cell between a member of the spacer frame and the central member. In another embodiment, the manifold is formed by a tube which is drilled with a plurality of calibrated holes and which is placed transversely in the upper part of the evaporation zone.
As an example, the anti-collapse device is a very wide-celled corrugated mesh. Preferably, a porous mat is applied to the inside of the evaporation zone, against the central member, and covers the surface which includes the communicating holes. The elementary cell also preferably includes a turbulence-promoting and falling-film-stabilizing structure.
A falling-film distillation unit based on films running down multiple-effect plates, in accordance with the present invention, includes at least one elementary cell having the above characteristics.
In a preferred embodiment, the distillation unit of the present invention includes a set of elementary cells assembled in thermal series. The condensation zone of a cell (n) heats the evaporation zone of a cell (n+1) of the thermal series. The evaporation zone of the first cell of the series is heated by a heating-cell through which an external heat-transfer fluid passes, and the condensation zone of the last cell of the series is cooled by a cooling cell through which a coolant flows. The elementary cells of a thermal series are mounted alternately, in two parallel planes.
In another preferred embodiment, a distillation unit is comprised of a set of cascades assembled in parallel. Each cascade includes elementary cells assembled alternately, in thermal series, in parallel assembly planes. As a result, each condensation zone of a cell (n) serves to heat the evaporation zones of two cells (n+1) placed on either side, each cascade starting with a heating cell and terminating in a cooling cell. All of the heating cells are fed in parallel, via a heat-transfer fluid, and all of the cooling cells are fed in parallel, via a coolant circuit.
In a particularly preferred embodiment, a distillation unit is provided which has a single cell with an evaporation zone which is inserted between two heating cells and a condensation zone which is inserted between two cooling cells. The entire assembly is surrounded by a layer of heat insulating material.
Irrespective of the embodiment which is chosen, the cells can all be held together in sealed contact by threaded rods extending between external assembly plates and which can be pressed together, for example, by compression members.
The present invention is further directed to a process for operating a distillation unit comprising one or more evaporation zones, and one or more condensation zones. The evaporation zones and the condensation zones are arranged vertically. The liquid to be distilled enters the upper part of the evaporation zone. The vapors produced in the evaporation zone are transferred horizontally, into the condensation zone, through a communicating structure. The condensed liquid then runs vertically down the walls of the condensation zone and is evacuated from the lower part of the condensation zone. The condensation zones can be maintained under reduced pressure in order to assist the horizontal transfer of the vapors from one zone to another.
The invention will be more clearly understood from the description which is provided below, with reference to the following figures.